Earlier, we generated a conditional transgenic mouse line in which diphtheria toxin receptor was selectively expressed in mossy cells using the Cre/loxP system. Within one week after diphtheria toxin injection, we observed 80% loss of mossy cells throughout the longitudinal axis. We found no obvious or sustained epilepsy-like discharges in the hippocampus as measured by in vivo local field potential recordings. Interestingly, no mossy fiber sprouting was detected by Timm staining. These results suggested that, in contrast to previous reports showing that lesions of the entire hilar region induce massive mossy fiber sprouting and epilepsy, selective in vivo elimination of mossy cells does not trigger behavioral epilepsy or mossy fiber sprouting. This year, we found that dentate granule cells in the DT-treated mutants became hyperexcitable to afferent stimulation in in vitro slice preparation, and during this hyperexcitable state deficits in contextual pattern separation were detected. We also evaluated the immediate-early gene (IEG) expression in response to kainic acid (KA) injection under the assumption that an excitatory stimulus would cause more granule cells to discharge and activate IEG expression in mutants compared to controls. KA injection evoked Zif268 expression in more granule cells in mutants than in controls. We also examined the KA-induced seizure intensity. The cumulative seizure score of mutants for the hour following KA injection was significantly higher than controls. Together, these results all suggested an increase in granule cell excitability following mossy cell ablation. In summary, we concluded that mossy cell loss in vivo renders the granule cells hyperexcitable. Contrary to the predicted epileptogenesis implicit in the dormant basket cell hypothesis, however, it was insufficient to trigger the mossy fiber sprouting and epileptic discharges. Perhaps, in addition to the loss of mossy cells, neurodegeneration of other limbic areas, such as entorhinal cortex, is necessary to induce medial temporal lobe epilepsy. These findings provide new insights into the mechanisms of epileptogenesis in the limbic cortex. This year, for the manuscript forth revisions, To analyze the impact of mossy cell loss on in vivo brain activity in the dentate gyrus, we placed electrodes in the dentate gyrus to record local field potentials (LFPs). In comparison with the same animals/electrodes/behavioral state before and 4 weeks after DT treatment, LFP oscillatory powers at theta frequency (712 Hz) were enhanced during exploration in mutants one week from DT exposure. That no such changes occurred in DT-treated fDTR controls suggests that the transient increase in theta power in mutants is not due to DT treatment. DT injection shows no effect on LFP power spectra during immobility periods regardless of genotype. Since the theta input to the dentate gyrus in vivo is conveyed from entorhinal cortex by the perforant path to granule cells, transient elevation of theta oscillatory power may reflect a transient increase in granule cell excitability. This result, again, support our conclusion that dentate mossy cells play overall an inhibitory role in granule cell activity. The manuscript has finally been published (Jinde S, Zsiros V, Jiang Z, Nakao K, Pickel J, Kohno K, Belforte JE, Nakazawa K. (2012) Hilar mossy cell degeneration causes transient dentate granule cell hyperexcitability and impaired pattern separation. Neuron 76:1189-2000). We also published a review article on this subject, entitled Hilar mossy cell circuitry controlling dentate granule cell excitability (Jinde S, Zsiros V, and Nakazawa K (2013) Frontier Neural Circuits 7:14).